Shortening in frosting

If you are decorating a cake using a piping bag with your frosting, for roses or other details, a frosting that contains some shortening provides stability and will have crisper edges than an all butter frosting. Butter warms quickly in a pastry bag, so an all butter frosting can make decorating difficult as your hand warms the bag. Additionally, if you are putting your cake out at room temperature and it is hot and humid in the space, an all butter frosting can get very soft quickly. That said, it is fine to leave shortening out and use all butter if you can keep your cake out of a hot room. And if you want to try to decorate with an all butter frosting, just keep putting the pastry bag in the fridge as it gets warm.